testsuite: ensure $(ghciWayFlags) can be overridden

This commit revises boilerplate.mk in testsuite as well as a few other
places, to ensure the tests that do make use of $(ghciWayFlags) can
receive the right $(ghciWayFlags) from testsuite driver config.

(cherry picked from commit 78c8b900)
(cherry picked from commit 6d0cdeed)